In places that are already dense and have no room for further sprawl, the cost of building in ways that add capacity can be quite high. It's not just about the

In places that are already dense and have no room for further sprawl, the cost of building in ways that add capacity can be quite high. It's not just about the raw materials. It's also about labor, safety, regulations (many of which are there for good reasons like preventing people from dying in fires due to cheap construction or lack of escape routes). Even the cost of trucking in the materials and paying for parking tickets/getting permits to inconvenience the neighbors for months on end can be a major expense.